Bimorph mechanical structures are employed in various conventional applications. For example, coiled bimetallic strips act as an actuator in conventional thermostats. In another example, thermostatic faucets are employed to prevent scalding. In a further example, nichrome wire-wrapped bimetallic strips are used in automobile flashers that “blink” to signal a turn. Although bimorph mechanical structures exhibit a low intrinsic temperature sensitivity, a large enough temperature difference and a high enough aspect ratio may result in large bending motions.
However, there are many applications wherein a standard bimorph film is inadequate. For example, in biomedical devices, only a few degrees separate core temperature from skin temperature. Accordingly, this temperature difference may be too small to trigger a bimorph control. Additionally, since conventional bimorph mechanical structures respond to temperature in a linear manner, there are instances where low-level heat prematurely warps the bimorph, causing unintended behavior (e.g., a gap in a valve seat which causes a leak, rather than a seal).
Furthermore, bilayer films convert small differences in thermal expansion into large structural motion, at the expense of a reduction in force. In this regard, the displacement is proportional to the difference in expansion coefficient between the two layers. However, among common engineering materials (e.g., polymers and alloys), there is a relatively narrow range of available properties that provide robust materials with large intrinsic expansion coefficients.
In many cases, a bimorph structure's symmetric temperate response (negative or positive curvature below or above the “layflat” temperature) can be problematic.
For example, an “instinctive” air vent to control a hot-air damper may include a polyethylene terephthalate (PET)/polyethylene (PE) bimorph structure that acts as a flapper valve. Below a fixed temperature, the bimorph structure should lay flat, thereby sealing the vent, while opening wide at temperatures above the fixed temperature. Unfortunately, the continuous curvature with temperature makes tight sealing problematic. Thus, there is a need for a temperature-responsive bimorph structure and apparatus that activates over a narrow range of temperatures.
While embodiments are described herein by way of example for several embodiments and illustrative drawings, those skilled in the art will recognize that the embodiments are not limited to the embodiments or drawings described. It should be understood that the drawings and detailed description thereto are not intended to limit embodiments to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope as defined by the appended claims. The headings used herein are for organizational purposes only and are not meant to be used to limit the scope of the description or the claims. As used throughout this application, the word “may” is used in a permissive sense (i.e., meaning having the potential to), rather than the mandatory sense (i.e., meaning must). Similarly, the words “include,” “including,” and “includes” mean including, but not limited to.